Waterborne, or water based, polyurethanes have been in existence for a number of years and there has been steady improvement in the performance properties of these materials to fit specific needs. Aqueous polyurethanes have been commercially attractive for a number of reasons, one of the most important of which is the elimination or substantial reduction of solvents and volatile organic compounds (VOC) emissions into the atmosphere. Another important reason relates to the performance of aqueous polyurethane systems which is now comparable to or better than conventional solvent-based polyurethanes for many applications.
Among the important advancements in this area have been water based and substantially solvent free, two component polyurethane-polyurea dispersions, also known simply as two component (2K) aqueous polyurethane dispersions. These dispersions are superior to well known one component aqueous dispersions of polyurethanes in which the polyurethanes are typically first formed in a solvent based system. The two component water based polyurethanes also match or exceed performance of two component solvent based polyurethane coatings while minimizing emission of organic solvents.
Two component waterborne polyurethanes tend to dry more slowly than two component solvent borne polyurethane coatings. This enables the waterborne coating molecules to have more mobility in the coating for longer periods of time, until coalescence can occur. However, this longer open time allows particles of different pigments to segregate causing xe2x80x9cfloodingxe2x80x9d/xe2x80x9cfloatingxe2x80x9d at the surface.
Presently, there are aqueous acid functional polyester polyol compositions which can be reacted with conventional isocyanates, such as either hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI) trimer. When such two component systems contain two different pigments, they exhibit a flooding and/or floating problem and do not respond to typical approaches in solving flooding/floating behavior such as listed in Clive Hare""s book, Protective Coatings Fundamentals of Chemistry and Composition. On pages 456-457, the author describes xe2x80x9cfloatingxe2x80x9d behavior as mottled or blotchy color variations in the dry coating caused by one or more of the following differences: particle sizes. two types of pigments, extent of pigment flocculation, surface tension, density or temperature gradients formed by currents as the paint dries. On the other hand, xe2x80x9cfloodingxe2x80x9d causes a uniform color on the coating surface but variations in color throughout the coating thickness caused by different rates of pigment settling in the applied film. These various rates can be due to differences in pigment size, density, shape, or degree of flocculation. Flooding can be affected by temperature, and humidity and intensified by low viscosity. slow dry times and thick films. Some traditional solutions for eliminating flooding/floating behavior, as described in Clive Hare""s book, are to use low surface energy flow agents or thixotropes, to match pigment sizes used in dispersions, to normalize degree of pigment dispersion, or to decrease dry time.
Unfortunately, with the water based, two component polyurethane-polyurea systems based on aqueous acid functional polyester polyols, these traditional approaches were not successful in eliminating the flooding behavior.
Water based, two component polyurethane-polyurea systems are described in the following patents:
U.S. Pat. No. 5,508,340 discloses a substantially solvent free, aqueous two-component coating composition in which the first component is an aqueous polyol phase containing an acid-containing polyol combined with an amine phase containing one or more amines having active hydrogens that are reactive with isocyanate groups. The second component is one or more polyisocyanates. Example 9 shows a black pigmented solution prepared using 100 wt parts aqueous polyol/amine, 25 parts acid functional acrylic resin, 20 parts water, and 10 wt parts black pigment dispersion. The aqueous polyol/amine comprises an acid-containing polyol, a nonacidic aromatic polyester diol, trimethylol propane and an amine blend. 100 parts of this black pigmented solution is then reacted with 50 parts of HDI trimer at NCO/OH=2/1. Since there is only one type of pigment present in the formulation, no apparent or visual flooding or floating would be evident.
U.S. Pat. No. 5,552,477 discloses a binder for a coating system which consists of a polyol component and a polyisocyanate component. The polyol component consists of a high molecular weight (Mn greater than 500) acid containing polyol which is water dilutable and possesses hydroxyl groups, and a reactive diluent containing a low molecular weight polyol (Mn less than 500) which has at least one group that is reactive with isocyanates.
U.S. Pat. No. 4,120,841 discloses an acid functional acrylic copolymer for use in aqueous paints as a levelling agent to primarily eliminate craters and improve gloss, but to also prevent or decrease flooding by increasing wettability of the pigments.
J. R. Taylor and H. Foster, xe2x80x9cThe Pigmentation of Acrylic Resinsxe2x80x9d, JSDC December 1969, pp 579-588, recommends eliminating flooding and floating in solvent borne acrylic coatings by the addition of small amounts of silicone fluid (M.S. 200) or incorporation of a silicone treated calcium carbonate during the mill charge. The pigment particle size and surface treatment will also greatly affect degree of flocculation, therefore affecting flooding and floating. It is critical in coatings formulated using waterborne acrylic dispersions that the correct surfactants are chosen in the pigment dispersions. To ensure optimum color stability in the paint, surfactants with suitable HLB (hydrophilic lipophilic balance) values for the pigments used must be chosen.
P. Quednau, xe2x80x9cHow polymeric dispersants can reduce pigment paste stocksxe2x80x9d, Paint and Ink International, January/February 1995, p 9 states that a major problem in the coatings industry is the flooding and floating behavior of some pigment dispersions. Pigment concentrates based on high molecular weight polymeric dispersants, either polyurethane or polyacrylate based, are recommended to achieve good pigment stability without flooding and floating. Both types of dispersants contain tertiary amines leading to strong adsorption on the pigment surfaces due to their basic character. These dispersants need to have a high molecular weight of 10,000-25,000 to be effective in steric hindrance and deflocculation of the pigment particles.
U.S. Pat. No. 4,978,708 discloses aqueous-based basecoat compositions comprising an anionic polyurethane principal resin and an anionic acrylic pigment grind resin. The coating compositions are alleged to exhibit good pigment wetting and dispersion characteristics with improved shelf life and color stability.
U.S. Pat. No. 3,578,635 discloses certain halogenated carbon blacks as pigments in the preparation of paint and lacquer compositions which possess excellent non-flooding properties and excellent tinting properties.
This invention is directed to a blend of an aqueous polymer emulsion with an aqueous pigmented polyol/amine composition and to its use in the production of a substantially solvent free, aqueous, pigmented two component polyurethane-polyurea coating composition. The pigmented polyol/amine composition contains at least one acidic polyol and at least one non-acidic polyol together with one or more amines and two or more different pigments, all in water. One or more reactive polyisocyanates can be combined with the aqueous mixture of the polymer emulsion and the pigmented polyol/amine composition to form a water based, substantially solvent free, pigmented two component polyurethane-polyurea dispersion which is useful in coating systems and exhibits substantially reduced or no pigment flooding or floating.
The polymer emulsion should be compatible with the pigmented polyol/amine composition. The polymer emulsion is used in an amount sufficient to afford a thixotropic mixture when mixed with the polyol/amine composition, preferably to afford a thixotropic mixture when mixed with the pigmented polyol/amine composition. The thixotropic mixture should be compatible with the polyisocyanate and ideally yield a thixotropic composition.
It was unexpectedly found that. by adding the polymer emulsion to the pigmented polyol/amine composition, the flooding problem associated with the two pigments in a water based, pigmented two component polyurethane-polyurea dispersion was substantially reduced or even eliminated.
The aqueous polymer emulsion can comprise a polymer or mixture of polymers, preferably having a Tg greater than ambient temperature; i.e., greater than about 20xc2x0 C. Suitable polymer emulsions for use in this invention include, for example, aqueous emulsions, or dispersions, of acrylics; copolymers of acrylics with styrene, substituted styrene, butadiene and/or acrylonitrile; vinyl acetate polymers and copolymers; ethylene copolymers with one or more other monomers such as vinyl acetate and vinyl chloride; alkyd resins; epoxies; polyurethanes and polyesters including polyester polyols. The polymer of the aqueous polymer emulsion preferably contains acid functionality such as resulting from the incorporation of acid functional monomers into the polymer.
The polymer emulsion should be compatible with the pigmented polyol/amine composition. Compatibility of the aqueous mixture of pigmented polyol/amine composition and the polymer emulsion can be determined by various methods. For example, visual observation of a compatible blend of polyol/amine and polymer emulsion will show little or no coagulation, gelation, or separation for an extended period of time, e.g.,  greater than one year at RT. Liquid specimens can be maintained in a closed container and evaluated periodically for separation, gelation, and/or coagulation.
The preferred aqueous polymer emulsions, or latexes, of this invention comprise polymers based on the free radical polymerization of about 10 to 100 wt % of one or more acrylic or methacrylic monomers and 0 to 90 wt % of one or more other monomers. Preferred emulsion polymers are those based on 50 to 100 wt % of one or more acrylic monomers and 0 to 50 wt % of one or more other monomers. Acrylic monomer, as used here, refers to acrylic acid (AA) and methacrylic acid and their salts and esters. Examples are acrylic acid and/or methacrylic acid, methyl acrylate, methyl methacrylate (MMA), ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate (BA), n-butyl methacrylate, and 2-ethylhexyl acrylate. It is preferred that the polymer contain polymerized units of acrylic and/or methacrylic acid at  greater than 0 to 90 wt %, preferably 10 to 30 wt %.
Examples of other polymer latexes, or dispersions, are those based on polyvinyl acetate and its copolymers; copolymers of ethylene and vinyl chloride and/or vinyl acetate; copolymers of acrylic or methacrylic acid or esters with styrene or substituted styrene, butadiene, and acrylonitrile; alkyds; urethane; epoxies; polyurethanes and polyesters.
It is also contemplated that the aqueous polymer emulsion can be replaced with a redispersible polymer powder since the polyol/amine compositions contain water.
Appropriate polyol/amine compositions for use in this invention are described in detail in U.S. Pat. Nos. 5,352,733 and 5,508,340 which are hereby incorporated by reference.
The polyol portion of the polyol/amine composition comprises one or more acid containing polyols and one or more non-acid containing polyols which provides a hydroxyl functionality of at least 1.5; preferably 2, and an acid number of between about 15 and 200. By acid containing polyol is meant acidic polymeric polyols which will provide the overall polyol portion with an acid number of between about 15 and 200. Preferred acid containing polyols are polyester polyols containing a carboxylic acid or sulfonic acid group. The polyester polyols can be prepared by the esterification of organic polycarboxylic acids or anhydrides with organic polyols. Usually, the polycarboxylic acids and polyols are aliphatic or aromatic dibasic acids and diols but trifunctional acids and tri- or higher functional polyols may be utilized. The polyol portion can contain a variety of components or blend of components provided the stated criteria for hydroxyl functionality and acid number are met. The invention is not to be restricted to any particular acidic polyol inasmuch as combinations of acidic polyols may be utilized. Non-acid containing polyols include low molecular weight diols, triols and higher alcohols, low molecular weight amide-containing polyols and higher polymeric polyols such as polyester polyols, polyether polyols and hydroxy-containing acrylic copolymers.
A variety of amines or blends of amines can be utilized as long as the amine phase, or component, contains active hydrogens reactive with isocyanate groups such that the average active hydrogen functionality of the overall amine composition is at least 1.5 or greater and the amines are present in an amount that substantially neutralizes the acidic functionality of the polyol. Suitable amines or blend of amines comprise primary and secondary aliphatic, cycloaliphatic and aromatic amines; tertiary aliphatic amines; alkanol amines, dialkanol amines, trialkanol amines; polyamines such as oxyalkyleneamines; and mixtures thereof. Ammonia and amines not having an active hydrogen or having insufficient active hydrogens, e.g., tertiary alkyl amines, can be utilized for their neutralizing ability provided the overall amine composition has an average active hydrogen functionality of at least 1.5. Primary and/or secondary amines are preferred.
To formulate the polyol/amine composition, the acidic and non-acidic polyols of the polyol phase are preferably blended in a mixing tank with water at ambient temperature, the amines being present in the polyols or the water, resulting in an overall aqueous polyol/amine composition having a xe2x80x9csolidsxe2x80x9d, or actives, content of between about 33 and 95 wt %. The mixture can contain any desired additives such as defoaming agents, surface tension reducing agents, agents to regulate pH, etc. Preferred aqueous polyol/amine compositions are marketed as ADURA(copyright) 100 and 200 polyols by Air Products and Chemicals, Inc.
The pigmented two component polyurethane coating composition of the invention comprises a two pigment system. xe2x80x9cTwo pigmentxe2x80x9d system means at least two pigments differing in color. Suitable pigments for use in the coating compositions are those pigments known in the polyurethane coating art and include, for example, titanium dioxide, carbon black, red iron oxide, yellow iron oxide, black iron oxide, phthalocyanine blue, phthalocyanine green. zinc phosphate or other anticorrosive pigments, benzidine orange, benzidine yellow, carbizole violet, chrome yellow, molybdate red, cobalt blue or green and other organic pigments such as Monozo-based and diazo-based colors. The amount of pigment ranges from  greater than 0 to 50%, preferably 13 to 30%, based on pigment volume concentration (PVC).
A preferred method for preparing the modified aqueous pigmented two component polyurethane-polyurea dispersion is to first blend the polyol/amine dispersion with the polymer emulsion and water and then add the pigments. An effective blend of polymer emulsion and polyol/amine composition contains sufficient polymer emulsion to afford a thixotropic mixture, such effective amounts being about  greater than 0 to 50 wt % aqueous polymer emulsion and  less than 100 to 50 wt % aqueous polyol/amine composition, preferably 5 to 20 wt % aqueous polymer emulsion and 95 to 80 wt % aqueous polyol/amine composition; or correspondingly, based on active components,  greater than 0 to 35 wt % emulsion polymer and  less than 100 to 65 wt % polyol/amine composition, preferably 5 to 15 wt % emulsion polymer and 95 to 85 wt % polyol/amine composition. xe2x80x9cActivexe2x80x9d components comprises the polymers, polyols and amines, i.e., compounds other than the liquid dispersing medium which comprises at least 85 wt %, preferably at least 95 wt %, water. Obviously, an all water medium is also included and most preferred.
It is preferred that the polymer emulsion first be preblended with the same or a different polyol/amine composition to afford a preblend comprising 80 to 95 wt % of the aqueous polymer emulsion. This preblend, which need not itself be thixotropic and in most cases will not be, is then added in sufficient amount to the polyol/amine composition to yield a thixotropic mixture. The amount of each component ultimately is dictated by the properties desired when the mixture is applied as a film after addition of the polyisocyanate and curing.
The organic polyisocyanate is any well known isocyanate in the art for polyurethane coatings. For example, it can be an aliphatic, a cycloaliphatic or an aromatic polyisocyanate, or mixtures thereof. Aliphatic or cycloaliphatic polyisocyanates are preferred. Trimers of diisocyanates are also preferred although other polyisocyanates can be used. Particularly preferred isocyanate adducts are those based on hexamethylene diisocyanate (HDI). Hexamethylene diisocyanate trimer is the preferred isocyanate material.
The two component polyurethane-polyurea dispersion is formed by blending at least 0.5 isocyanate equivalents for each active hydrogen equivalent of the pigmented polyol/amine component. Preferably, there will be between about 1.1 and 3 isocyanate equivalents for each active hydrogen equivalent of the pigmented polyol/amine component with 1.3 to 2.0 isocyanate equivalents being especially preferred. For heat cured systems, the preferred isocyanate to active hydrogen ratio is 1.0 to 1.3.
Optional additives, such as catalysts, flow control agents, and thickeners, are preferably added to the polyol/amine composition at the same time as the polymer emulsion; however they can also be added before or after addition of the isocyanate component.
Polyisocyanate can be added to the blend of pigmented polyol/amine and polymer emulsion by ordinary mixing at room temperature. This composition, when fully cured, produces a crosslinked film having properties which are equal to or exceed solvent-based two component polyurethanes as well as other known aqueous-based two component polyurethanes with substantially reduced or eliminated floating/flooding of the pigments.
After addition of the polyisocyanate, the mixture has a limited pot life within which it should be applied to a substrate. Typically the pot life ranges from 1 to about 4 hours. The mixture is suitable for coating a variety of surfaces; for example, wood, concrete, plastic, masonry, and metallic substrates.
A preferred method for preparing the two component polyurethane coating composition comprises (1) preparing a white pigment grind, (2) adding the grind to the aqueous polyol/amine composition, (3) adding an aqueous preblend of acrylic polymer emulsion and a polyol/amine composition, (4) mixing in the additives, (5) adding the tints and (6) blending in the isocyanate.
The following examples demonstrate that the addition of an aqueous acrylic polymer emulsion to an aqueous polyol/amine-containing blue paint formulation significantly lowers flooding as measured by Delta E color change throughout the pot life. Delta E is a measure of the total color change and is defined as:
Delta E={(Delta L)2*(Delta a)2*(Delta b)2}0.5
where Delta L is the difference in lightness (black-white scale), Delta a is the difference in the red-green scale, and Delta b is the difference in the blue-yellow scale. Delta E values of  less than 1 are typically undetectable with the naked eye, however Delta E values xcx9c5 or greater are extremely noticeable.
Acrylic Emulsionxe2x80x94a 36 wt % aqueous acrylic polymer emulsion, polymer comprises
MMA (48 wt %)/BA(32 wt %)/AA(20 wt %); Tgxcx9c47xc2x0 C.
Acrylic/Polyolxe2x80x9487/13 blend of Acrylic Emulsion and ADURA 100 polyol
ADURA 100 polyol (acid functional polyol/amine) from Air Products and Chemicals, Inc
Bayhydur XP7063 water dispersible isocyanate based on HDI from Bayer
Byk 348 surfactant from Byk-Chemie
Byk 380 surfactant from Byk-Chemie
DeeFo PI-4 defoamer from Ultra Additives
Desmodur N3300 HDI trimer from Bayer
DisperByk 190 surfactant from Byk-Chemie
Foamex 805 defoamer from Tego Chemie
K-Kat 6212A catalyst from King Industries
Surfynol 504 surfactant from Air Products and Chemicals, Inc.
TintAyd NV7292 pigment dispersion from Elementis Specialties
Tinuvin 1130 UV absorber from Ciba-Geigy
Tinuvin 292 light stabilizer from Ciba-Geigy
TiPure R960 TiO2 from DuPont